Assessment of purity and screening of peptide libraries by nested ion mobility-TOFMS: identification of RNase S-protein binders.

Combinatorial peptide synthesis in combination with affinity selection and high-resolution ion mobility/time-of-flight mass spectrometry (IM/TOFMS) analysis has been used to investigate the binding of a series of 96 related eight-residue peptides (with the general sequence NH2-GX1X2FX3X4X5G-CO2H, where X1 = L, F, V, Y; X2 = N, F; X3 = E, V, T; X4 = V, L; X5 = V, L) to the ribonuclease S protein. A key advantage of this strategy is that the IM/ TOFMS approach allows the relative abundances of individual library components (including numerous sequence and structural isomers) to be characterized before and after screening. The relative binding interactions of different sequences are assessed by comparing IM/TOFMS data for those components that pass through the column (as well as those that bind) to data for the library prior to screening. The high-affinity sequences that are found in this study are compared with those selected from much larger combinatorial libraries. The results suggest that many expected sequences in the large libraries may be missing (e.g., due to issues such as failure of specific steps during the synthesis or differences in solubility). Comparison of the binding sequences obtained in these studies and those reported previously indicates that screening results from large libraries should be interpreted with caution.